The provision of the founding treaties in which duty of loyal (sincere) cooperation of Member-States with Community institutions is stipulated, together with the correlating duty of abstention from measures that may jeopardize attainment of aims of the founding treaties, has been the subject of interpretation of the European Court of Justice since the Rome Treaty until present-day, having served as the grounds for numerous and material improvements and build ups of the Community legal system, which encompassed the principle of primacy of Community law, implied exclusive competence of the Community in external relations, the principle of effectiveness of Community law, etc. It appears that only case law of the ECJ in the future may provide answer to the question whether there is a single principle of loyalty, or simply a bundle of related duties. If one accepts that it exists, the principle of loyalty seems most closely related to the principle of autonomy of EU law, both judging by its importance for the development of the legal system of the EU, and by the fact that it directly borders the political sphere.